1. Field of the Invention
The present invention relates generally to measuring the optical signal-to-noise ratio (OSNR) in an optical node of an optical network. More particularly, the present invention is directed towards monitoring the OSNR in-service.
2. Description of Background Art
The optical signal-to-noise ratio (OSNR) is commonly used as a metric to characterize an optical path (or optical links within an optical path) of a signal of an optical network. Conventionally, an optical network is designed so that the OSNR at a receiver exceeds a selected minimum (threshold) OSNR. The threshold OSNR is commonly calculated using a power budget analysis technique. In a power budget analysis, optical signal at the receiver must have a minimum power level and a minimum OSNR to achieve a desired bit error rate (BER). The threshold OSNR typically depends upon the bit rate and the transmitter-receiver technology. For example, a forward error correction (FEC) encoding technique may reduce the OSNR required to achieve a selected BER.
In conventional dense wavelength divisional multiplexed (DWDM) optical networks, the OSNR is typically measured at selected node locations as part of testing during an initial set-up procedure. Referring to FIG. 1, a monitoring tap 105 may be arranged to couple a portion of one or more optical signals between two points 102 and 104 of an optical node. The two points may, for example, be the input and output ports of an optical amplifier 110 (e.g., an erbium-doped fiber amplifier, such as a pre-amplifier or a post-amplifier) for amplifying a plurality of optical wavelength channels. A test access port 115 is commonly connected to the monitoring tap 105 to permit an optical spectrum analyzer to be used to characterize the optical characteristics of the tapped signal(s) during testing. The optical spectrum analyzer may, for example, include a grating monochrometer to scan the optical power as a function of wavelength. The OSNR of a wavelength channel may be calculated by performing a spectral analysis of its noise power. Referring to FIG. 2, the noise power of the channel may be calculated by distinguishing the spectral properties of amplified spontaneous emission (ASE) noise 220 from the peak channel signal 215. The ASE noise power can be estimated by interpolating values of the ASE noise 220 over the signal wavelengths and integrating the interpolated ASE noise function (as indicated by the hatched region 225). The signal power level can then be calculated by subtracting the noise power from the total power in the channel.
The measurement of OSNR in-service is desirable in optical networks, particularly in dynamically re-configurable networks. In particular, dynamically configurable networks using multi-protocol lambda switching (MP(lambda)S) have been proposed that could benefit from in-service OSNR monitoring. In some MP (lambda)S approaches, the quality of service (QOS) of an optical path is an important consideration in routing MP(lambda)S data packets. Since the OSNR is an important parameter that limits the QOS, a cost-effective technique to measure the OSNR of every channel in each node of an optical network is of interest for dynamically configurable networks, such as MP(lambda)S networks.
Unfortunately, conventional techniques to monitor OSNR are expensive. A dedicated multi-channel optical spectrum analyzer capable of simultaneously monitoring every channel in a node is prohibitively expensive and has other limitations, such as speed limitations associated with using a physical grating. In principle, each de-multiplexed wavelength channel of a DWDM node may be coupled to a single-channel OSNR monitoring apparatus that analyzes the optical spectrum of one wavelength channel. For example, a single-channel OSNR monitor may use a combination of optical elements and software to perform a spectral power analysis within a selected wavelength range. However, a conventional single-channel OSNR monitor is typically expensive and may lack the desired resolution. For example, a single-channel OSNR monitor using a digital signal processing technique to analyze the noise spectrum may require a dedicated digital signal processing microprocessor to analyze the frequency components of the signal. However, since a DWDM node may have a substantial number of channels, the total node cost of employing a conventional single-channel OSNR monitor for each channel is prohibitively large.
What is desired is a new, less expensive apparatus and method to monitor the OSNR of one or more optical data channels in an optical network.